Hi,
I am a freelance web developer and mostly develop sites based on WordPress and I would never recommend you to use nulled themes or plugins for your client as most of nulled scripts contain malicious viruses and backdoor hooks so you got no clue when your client's personal data be stolen or site go down for no reason or for ransom so being a developer be careful and some good people above did explain that you should talk with your client upfront about the pricing and what your package includes and what not which is trust building. I personally highly discourage you to use any nulled script in a live site especially for a client or you will end up losing that precious client's trust as well as the money for onward services. Better use nulled scripts on your own server or localhost for practice purpose like i do just learn different popular themes, plugins, builders ... etc then when building a website for a client ask him you need to pay for such and such plugin or recommend him and most of the clients would simply pay not a problem for them and they love getting ideas from the developer itself, hope it helps dear.